The present invention relates to a method of measuring the concentration of hydrogen and methane in nitrogen by ion mobility spectrometry.
Nitrogen is widely employed as a reacting medium or carrier gas in the integrated circuit industry. As is known, in the manufacture of these devices, the purity of all the used materials has a basic importance. As a matter of fact, contaminants possibly present in the reactants or in the reaction environment may be incorporated into the solid state devices, thus altering their electrical features and giving rise to production wastes. The purity specifications of the gases employed in production may change among different manufacturers and depending on the specific process the gas is employed in. Generally, a gas is considered to be acceptable for manufacturing purposes when its impurities content does not exceed 10 ppb (parts per billion, namely an impurity molecule per 109 total gas molecules). Preferably, the impurities content is lower than 1 ppb. It thus becomes important to have the possibility to measure extremely low concentrations of impurities in the gases in an accurate and reproducible way.
A technique that can be exploited for such purpose is ion mobility spectrometry, which is also known in the art under the abbreviation IMS. The same abbreviation is also used for the instrument employed to perform this technique, while indicating, in this case, “Ion Mobility Spectrometer”. The interest in such a technique comes from its extremely high sensitivity, associated with limited size and cost of the instrument. By operating in suitable conditions, it is possible to detect gas or vapor phase species, in a gas medium, in amounts of the picogram order (pg, namely 10−12 g) or in concentrations of the order of parts per trillion (ppt, equivalent to one molecule of analyzed substance per 10−12 molecules of sample gas). IMS instruments and analytical methods in which they are employed are disclosed, for instance, in U.S. Pat. Nos. 5,457,316 and 5,955,886, assigned to the US company PCP Inc.
The physicochemical grounds of the technique are very complicated, just as the interpretation of the IMS analytical results. For an explanation of these grounds and results, reference can be made to the book Ion Mobility Spectrometry by G. A. Eiceman and Z. Karpas, published in 1994 by CRC Press.
Briefly, an IMS instrument essentially consists of a reaction zone, a separation zone and a collector of charged particles.
Within the reaction zone takes place the ionization of the sample, comprising gases or vapors to be analyzed in a carrier gas, usually by means of β-radiation emitted by 63Ni. The ionization mainly occurs on the carrier gas, with the formation of the so-called “reactant ions,” whose charge is then distributed to the present species depending on their electron or proton affinities or on their ionization potentials.
The reaction zone is divided from the separation zone by means of a grid which, when maintained at a suitable potential, prevents the ions produced in the reaction zone from entering into the separation zone. The analysis “time zero” is established by the moment when the grid potential is annulled, thus allowing the ions admission into the separation zone.
The separation zone comprises a series of electrodes which create such an electric field that the ions are carried from the grid towards the collector. In this zone, maintained at atmospheric pressure, a gas flow is present having an opposite direction with respect to that of the ions' movement. The counterflow gas (defined in the field as “drift gas”) is an extremely pure gas, that may either correspond to the gas whose impurities content is to be determined, or may be a different gas. For instance, for determining the impurities content in nitrogen it is either possible to use a counterflow of pure nitrogen or argon, as disclosed for instance in Italian patent application MI2000-A-002830, assigned to SAES Getters, S.p.A. The motion velocity of the ions depends on the electric field and on the cross-section of the same ions in the gaseous medium, so that different ions take different times for crossing the separation zone and reaching the particles collector. The time elapsed from “time zero” to the time of arrival on the particle collector is called “time of flight.” The collector is connected to the signal processing system, which transforms the current values sensed as a function of time in the final graph, where peaks corresponding to the different ions are shown as a function of the “time of flight.” From the determination of this time and the knowledge of the test conditions, it is possible to trace the presence of the substances, which are the object of the analysis, whereas from the peaks' area it is possible to calculate, through suitable computation algorithms, the concentration of the corresponding species.
In order to get good results from such a type of analysis, it is necessary that the peaks corresponding to the various species be well distinct and separated from one another. In a few cases, however, this condition is not satisfied. That is what happens, for example, in the case of the analysis of methane as an impurity in nitrogen. Within the reaction zone methane dissociates into H+ ions and CH3−. radicals (these latter are not “visible” in the IMS analysis as being neutral species). As a consequence, it is impossible to measure methane whose hydrogen concentration is higher than that actually present in nitrogen at the instrument's inlet.